EEs take optical aim at ultrawideband RF

Portland, Ore. - An electrical engineer from Purdue University claims that his team has developed the first all-optical method of generating ultrawideband signals.

The future of UWB communications is bright, promising everything from high-speed wireless communications to automobile collision avoidance to personal-area networks to ground-penetrating radar to imaging systems that can see through walls.

Spurred by the promise of such applications, researchers are hard at work on the Federal Communications Commission's UWB slot from radio frequency (RF) 3.1 to 10.6 GHz.

A Purdue University (West Lafayette, Ind.) professor claims to have brought commercial UWB one step closer by shaping the UWB signal within an all-optical modulator.

"Our enabling tool is optical pulse shaping," said Purdue visiting EE professor Jason McKinney. "You simply cannot create our waveforms electronically-certainly not programmatically." McKinney said he is building on the previous work of his co-researcher at Purdue, EE professor Andrew Weiner, who perfected the method used here to shape the optical spectrum. Ingrid Lin, a Purdue doctoral student, also contributed to the work.

Normally, UWB signals are created with electronics. Very short pulses are generated, sometimes as simple as a single sinusoidal pulse at the desired frequency or with various other burst and pulsed shapes. In contrast, the Purdue method is to shape the frequency spectrum of an optical signal, then Fourier transform it to the time domain with a 5.5-kilometer fiber cable that imposes a frequency-dependent linear time delay. The cable then illuminates a photodiode. This results in direct, simultaneous modulation of the optical spectrum and its output UWB waveform. With this method, the Purdue researchers claim to be able to craft any shape of waveform-from sinusoidal to one specifically adapted to cancel multipath.

"What we do is pattern the optical spectrum in a Fourier-transform pulse shaper, so that the output of that, in time, is a Fourier-transform relation between that spectral patterning and the optical pulse," said McKinney. "When you put that through a long length of optical fiber, the wavelength-dependent propagation velocity effectively maps the shaped optical power spectrum to a time-domain optical waveform. Then when you shine that on a photodiode, what you get in time is an electrical waveform that has the same patterning as the optical spectrum."

The Purdue professors also claim they can precisely tailor a UWB waveform's spectrum so that any future changes in the FCC mandates can be retroactively implemented, such as a notch at 9 GHz. Its real-time adaptive signal shaping also enables it to craft distortion-free signals, according to the researchers.

"The key difference in this approach is that it runs open-loop, and thus can operate in real-time without iteration," said McKinney.

Photonic synthesis of broadband RF waveforms for UWB is so fast to adapt, according to the researchers, because it does not depend on feedback. By running an open-loop reflection-mode dispersive Fourier transform, the optical pulse shaping and subsequent frequency-to-time conversion are performed in real-time. The researchers demonstrated burst, monocycle and pulsed waveforms with precisely tailored RF bandwidths from 1 to 8 GHz. Temporal durations were as short as 200 picoseconds.

Next, the researchers plan to apply the technique to rid signals of unwanted effects that are impossible to cancel today without photonics. For example, the researchers are currently attempting to screen out interference in ground-penetrating radar (GPR) so that individual land mines can be imaged precisely. Today's GPR can only image a general area.

"If two items are buried close together, then a pulse that is too long will give you a combined reflection from both items, but if your pulse is short enough, you can get a separate reflection from each," said McKinney.